1. Field of the Invention
This invention relates to a process affording high activity for disproportionation of toluene utilizing a crystalline aluminosilicate zeolite catalyst modified with phosphorus as a result of treatment with an ammonium hydrogen phosphate to yield benzene and xylenes in which the proportion of para-xylene isomer is substantially in excess of its normal equilibrium concentration.
2. Description of the Prior Art
The disproportionation of aromatic hydrocarbons in the presence of zeolite catalysts has been described by Grandio et al. in the Oil and Gas Journal, Vol. 69, Number 48 (1971).
U.S. Pat. Nos. 3,126,422; 3,413,374; 3,598,878; 3,598,879 and 3,607,961 show vapor phase disproportionation of toluene over various catalysts.
U.S. Pat. No. 4,011,276 discloses disproportionation of toluene to produce benzene and xylenes rich in the para isomer with a catalyst comprising a crystalline aluminosilicate zeolite of the ZSM-5 type which has been modified by the addition thereto of a minor proportion of an oxide of phosphorus and a minor proportion of an oxide of magnesium. U.S. Pat. No. 4,016,219 discloses similar reaction but wherein the catalyst employed has been modified by the addition thereto of phosphorus in an amount of at least about 0.5 percent by weight. While the processes of each of these patents show selective production of para-xylene, such was obtained only by tolerating a very substantial reduction in toluene conversion, i.e., decrease in activity compared to disproportionation of toluene carried out under comparable conditions with the unmodified zeolite catalyst.
In these prior art processes, the xylene product produced either has the equilibrium composition of approximately 24 percent of para, 54 percent of meta and 22 percent of ortho or in those instances where the para isomer has been produced in an amount in excess of its equilibrium concentration, such was achieved only at great expense of activity, i.e., a very substantial reduction in toluene conversion. Of the xylene isomers, i.e., ortho-, meta- and para-xylene, meta-xylene is the least desired product, with ortho- and para-xylene being the more desired products. Para-xylene, in substantial yield, is of particular value being useful in the manufacture of terephthalic acid which is an intermediate in the manufacture of synthetic fibers such as "Dacron". Mixtures of xylene isomers either alone or in further admixture with ethylbenzene have previously been separated by expensive superfractionation and multistage refrigeration steps. Such process, as will be realized, has involved high operation costs and has a limited yield.